This class represents a handle on a stack of activities.
It should only be used for suspending and resuming the
thread-to-activity association.
The activity at the top of the stack is the current activity.

Although users won't typically care what the underlying implementation
of a context is, they will need to do comparisons.
So, although this method is provided by Java.Object we have it here
to ensure that we don't forget to implement it!